Prior U.S. Pat. No. 4, 291,740 to Michelson, and related U.S. Pat. Nos. 4,232,726 and 4,311,184, describe a process for making foundry shell cores or molds, referred to as a heatless process.This process vibrates a sand/binder mix into a generally cylindrical cavity formed by a pair of gas permeable, concave patterns pressed together. A catalyst gas is then forced into the cavity through the pattern walls, causing polymerization of the binder, and, thus, hardening of the sand/binder mix. The thickness of the layer of hardened mix depends upon the pressure of the catalyst gas employed and amount of compression of residual air in the cavity. After curing the catalyst contacted mix for a predetermined period of time, the remaining unhardened center of loose sand and unreacted binder is removed by turning the pattern box upside down and blowing out the interior with compressed air. The pattern is then opened and the resulting sand core or mold is removed.
The present invention is concerned in part with the preparation of such patterns.
In the Michelson process, as disclosed in the U.S. Pat. No. 4,291,740 patent, the patterns are made of iron or steel sheet metal with a plurality of permeable inserts positioned in the wall of the patterns in a chessboard-like, staggered arrangement. The inserts may be of the screen type.
One problem with this arrangement is that the filters tend to plug up with sand/binder mix. In addition, the use of spaced openings does not allow or provide a continuous gas permeable surface for uniform treatment of sand/binder mix.
In the U.S. Pat. No. 4,291,740 patent, it is mentioned that the patterns can be made of sintered powdered metal. Whereas such patterns are gas permeable all around and, thus, less likely to plug, such patterns, if made in complicated or irregular shape, tend to crack on outer diameters during manufacture. This is believed to be due to shrinkage during sintering. Sintering takes place at an elevated temperature which is about 2/3 of absolute temperature for the metal particular, and in the process there is a general tendency for the particulates of the metal to change shape and size and generally flow closer together. The mold used to form the pattern shape, however, does not shrink, with the result that cracks in surfaces restrained by the mold surfaces develop. In one example, in the formation of a truncated cone-shaped part between two graphite mold halves, the shrinkage during sintering was so severe as to provide a strong force tending to force the two mold halves apart.